The anionic amino acid glutamate is important as a substrate for gluconeogenesis, ureagenesis, and protein synthesis and as an acceptor for shuttling nitrogen via glutamine. In the liver, glutamate is a substrate for glutamine synthetase, an activity important in the hepatic "intercellular Glutamine Cycle", which is essential for whole body nitrogen homeostasis. This cycle relies on hepatocyte heterogeneity with regard to the expression of glutaminase and urea cycle enzymes versus glutamine synthetase, and on the heterogenous intercellular and intracellular expression of high-affinity glutamate transport activity. The developmental expression of hepatic glutamate transport capacity and the transporter proteins responsible for this activity has not been systematically explored in the liver. The recent cloning of the cDNAs and the generation of antibodies for the Na+-dependent anionic amino acid transporters (GLAST1, GLT1, EAAC1, and EAAT4) gives us, for the first time, the tools necessary to approach these issues at the molecular level. The objective of this proposed research is to define the developmentally-regulated expression and localization of anionic amino acid transport proteins and activity in hepatocytes and to define their differential distribution in the acinar axis and plasma membrane subdomains of hepatocytes.